The immune system has evolved multiple regulatory mechanisms to protect the host from pathogens without the self-infliction of immune pathology through an unchecked response. There is now compelling evidence that a subset of T cells defined as CD4+CD25+ (Tregs), suppress T cell activation in response to both self and microbial antigen stimulation. We previously hypothesized that Tregs play a critical role in modulating T cell activation during HIV infection. In support of this hypothesis, recently, results from our lab and others showed that Tregs are disrupted during late stages of the disease and impair HIV-specific immunity during earlier stages of the infection. These findings suggest Tregs may act as a double-edged sword during HIV infection and understanding how they fine tune immune activation will likely reveal important clues to HIV pathogenesis. Many critical gaps remain in the immunobiology of human Treg cell development and function. It is also unclear whether Tregs specifically recognize HIV antigens and have a beneficial or harmful role during HIV infection. Based on our novel findings and innovative methods to reprogram naive T cells into Tregs, we are poised to embark on addressing a number of key questions concerning the mechanism of Treg development, function and susceptibility to HIV infection. To achieve these goals we propose to determine: 1) the activation parameters of human Treg development and expansion, 2) the molecular basis of Treg hyporesponsiveness to TCR stimulation;and using the tools and knowledge from the first two aims, 3) to define antigen specificity of Tregs and whether they suppress HIV-specific immune response. The proposed studies will yield an important cellular and molecular framework to understand the in vivo interplay between Tregs and HIV. It is also anticipated that results gained from this study will facilitate the rational design of new strategies to therapeutically manipulate the regulatory arm of the immune system. Indeed, downregulation of Tregs may prove to be highly beneficial in terms of bolstering the efficacy of vaccines to HIV and other human pathogens.